A creature attacks alone if it's the only creature declared as an attacker during the declare attackers step. For example, Tempered in Solitude's ability won't trigger if you attack with multiple creatures and all but one of them are removed from combat.

You must pay all costs and follow all normal timing rules to play a card this way. For example, you may play a land this way only during your main phase while the stack is empty, and only if you haven't yet played a land this turn.